The present invention relates to an adjustment of an oscillating frequency of a voltage-controlled temperature-compensated oscillator thanks to an external time or frequency reference.
In order to be able to clock processing operations performed by a device, a voltage-controlled temperature-compensated oscillator VCTCXO can be used. This type of oscillator is found in many items of communication equipment.
In the mobile telephony world, a femtocell is a cell in a mobile telephony cell system offering limited radio coverage and generally dedicated to domestic use. The base station, referred to as a femto, responsible for managing the femtocell, connects to the network of the mobile telephony operator via an Internet gateway. Such femtos typically have an oscillator of the VCTCXO type.
When a device uses a VCTCXO oscillator, it may be necessary to have recourse to an external time or frequency reference in order to adjust the oscillation frequency of the oscillator and thus allow complying with the required frequency precision, e.g. +/−250 ppb (parts per billion). For example the NTP (Network Time Protocol, as defined in the standard documents RFC 1305 and RFC 5905) and PTP (Precision Time Protocol, as defined by the IEEE 1588 standard) protocols should be noted, enabling to synchronise the oscillation frequency of the oscillator thanks to an external time reference.
The frequency stability of a VCTCXO oscillator depends essentially on the variation in the ambient temperature around the oscillator and its ageing. The variations in ambient temperature may be rapid and considerable, e.g. several tens of degrees in less than one hour, and cause, over a period of one or two hours, the maximum variations in frequency provided for in the specifications of the oscillator. As for ageing, this causes smaller and slower variations in frequency, e.g. a few tens of ppb over several days, i.e. approximately 1 ppm (parts per million) over one year. The variations in frequency caused by the variations in ambient temperature are therefore the most difficult to correct in terms of frequency, or rate, of access to the external time or frequency reference.
In order to be able to ensure the required precision of the oscillator, it is usual to define the frequency, or rate, of access to the external time reference so as to be able to correct the most constraining predicted variations in frequency. This implies high data traffic between the device comprising the oscillator and the external time reference and reduces the maximum number of devices that a same time server can manage in parallel.
In order to limit the frequency, or rate, of access to the external time reference, it is known to use a VCTCXO oscillator that is sufficiently stable and/or pre-calibrated in terms of temperature so that exchanges with the time server acting as the external time reference are intended only for corrections due to the ageing of the oscillator. This approach does however pose a problem of manufacturing cost.
It is desirable to overcome the drawbacks of the prior art. It is in particular desirable to provide a solution that enables adjusting an oscillation frequency of a voltage-controlled and temperature-compensated oscillator thanks to an external time reference while preserving the network resources for accessing this external time reference, as well as the processing resources of the device implementing this external time reference.
It is in particular desirable to provide a solution that is simple to implement at low cost.